Method and apparatus for exhaust aftertreatment in an internal combustion engine

TECHNICAL FIELD

This disclosure relates to operation and control of internal combustion engines selectively operative lean of stoichiometry and associated exhaust aftertreatment systems.

BACKGROUND

The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.

Operating an internal combustion engine under lean of stoichiometry conditions can improve fuel efficiency, but may increase NOx emissions. Known aftertreatment systems for internal combustion engines operating lean of stoichiometry can include a three-way catalytic converter upstream of a lean-NOx reduction catalyst, also referred to as a lean NOx trap (hereafter ‘LNT device’), which can be used in concert with other exhaust aftertreatment devices. Known three-way catalytic converters (‘TWC’) function to oxidize engine-out hydrocarbons (‘HC’) and carbon monoxide (‘CO’), and reduce nitrides of oxygen (‘NOx’) emissions during stoichiometric engine operation and oxidize HC and CO emissions during lean operation.

A LNT device adsorbs NOx emissions during lean engine operation and preferably operates within about a 250° C. to 450° C. temperature range with effectiveness decreasing above and below that temperature range. Effectiveness of known LNT devices can be reduced due to exposure to elements present in fuel, including sulfur. A LNT device requires periodic regeneration to desorb and reduce adsorbed NOx elements. Regenerative techniques can include operating the spark-ignition engine at an air/fuel ratio that is rich of stoichiometry for a period of time.

During engine operation, an oxygen storage capacity (hereafter ‘OSC’) in the TWC can become saturated with oxygen during lean engine operation. During an initial portion of the regeneration process, oxygen stored in the TWC oxidizes reductants (e.g. HC, CO, H2) emitted by the engine and emits the processed mixture to the LNT device. This air/fuel mixture causes the LNT device to desorb stored NOx. As the reductants are consumed in the TWC during the initial portion of the regeneration process, desorbed NOx emissions may escape.

Known regeneration methods include running the engine rich of stoichiometry to generate reductants which react with stored NOx to regenerate the LNT. A significant portion of the reductants is consumed by the oxygen stored in the TWC. Additionally, the rich air/fuel ratio necessary to neutralize the oxygen stored in the TWC results in a delivery delay of reductants to the LNT further decreasing NOx reduction efficacy.

SUMMARY

An exhaust aftertreatment system is fluidly connected to an internal combustion engine and signally connected to a control module. The system consists of a three-way catalytic converter with an output exhaust tube, a flow diverter valve operatively connected to the engine and fluidly connected upstream from the three-way catalytic converter, an exhaust flow tube having a first end fluidly connected to the diverter valve and a second end fluidly connected to the output exhaust tube of the three way catalytic converter, a catalyzed NOx adsorber device fluidly connected to the output tube of the three way catalytic converter, and a NOx sensing device connected downstream of the catalyzed NOx adsorber device.

A method for managing an exhaust gas feedstream includes operating the engine and monitoring the exhaust gas feedstream. The exhaust gas feedstream is selectively directed through the three-way catalytic converter and through the catalyzed NOx adsorber device based upon the monitored exhaust gas feedstream. And, the exhaust gas feedstream is selectively diverted around the three-way catalytic converter to the catalyzed NOx adsorber device based upon the monitored exhaust gas feedstream.

BRIEF DESCRIPTION OF THE DRAWINGS

One or more embodiments will now be described, by way of example, with reference to the accompanying drawings, in which:

<figref idref="DRAWINGS">FIG. 1</figref> is a schematic drawing of an exemplary engine system, in accordance with the present disclosure;

<figref idref="DRAWINGS">FIG. 2</figref> is a flow chart that illustrates exhaust management during LNT regeneration, in accordance with the present disclosure; and

<figref idref="DRAWINGS">FIG. 3</figref> graphically illustrates CO breakthrough out of the TWC and NOx reduction efficiency for varying oxygen levels in the TWC, in accordance with the present disclosure.

DETAILED DESCRIPTION

Referring now to the drawings, wherein the depictions are for the purpose of illustrating certain exemplary embodiments only and not for the purpose of limiting the same, <figref idref="DRAWINGS">FIG. 1</figref> schematically shows an internal combustion engine <b>10</b>, an accompanying control module <b>5</b>, and an exhaust aftertreatment system <b>70</b> that have been constructed in accordance with an embodiment of the disclosure. The engine <b>10</b> may be selectively operative in a plurality of combustion modes, including a controlled auto-ignition combustion mode, a homogeneous spark-ignition combustion mode, and a stratified-charge spark-ignition combustion mode. The engine <b>10</b> is selectively operative at a stoichiometric air/fuel ratio and at an air/fuel ratio that is primarily lean of stoichiometry. The disclosure can be applied to various internal combustion engine systems and combustion cycles.

The exemplary engine <b>10</b> comprises a multi-cylinder direct-injection four-stroke internal combustion engine having reciprocating pistons <b>14</b> slidably movable in cylinders <b>15</b> which define variable volume combustion chambers <b>16</b>. Each piston <b>14</b> is connected to a rotating crankshaft <b>12</b> by which linear reciprocating motion is translated to rotational motion. An air intake system provides intake air to an intake manifold <b>29</b> which directs and distributes air into an intake runner to each combustion chamber <b>16</b>. The air intake system comprises airflow ductwork and devices for monitoring and controlling the air flow. The air intake devices preferably include a mass airflow sensor <b>32</b> for monitoring mass airflow and intake air temperature. A throttle valve <b>34</b> preferably comprises an electronically controlled device which controls air flow to the engine <b>10</b> in response to a control signal (‘ETC’) from the control module <b>5</b>. A pressure sensor <b>36</b> in the manifold is adapted to monitor manifold absolute pressure and barometric pressure. An external flow passage recirculates exhaust gases from engine exhaust to the intake manifold, having a flow control valve, referred to as an exhaust gas recirculation (‘EGR’) valve <b>38</b>. The control module <b>5</b> is operative to control mass flow of exhaust gas to the intake manifold <b>29</b> by controlling opening of the EGR valve <b>38</b>.

Air flow from the intake manifold <b>29</b> into each of the combustion chambers <b>16</b> is controlled by one or more intake valves <b>20</b>. Flow of combusted gases from each of the combustion chambers <b>16</b> to an exhaust manifold <b>39</b> is controlled by one or more exhaust valves <b>18</b>. Openings and closings of the intake and exhaust valves <b>20</b> and <b>18</b> are preferably controlled with a dual camshaft (as depicted), the rotations of which are linked and indexed with rotation of the crankshaft <b>12</b>. The engine <b>10</b> is equipped with devices for controlling valve lift of the intake valves and the exhaust valves, referred to as variable lift control (hereafter ‘VLC’) devices. The variable lift control devices are operative to control valve lift, or opening, to one of two distinct steps, e.g., a low-lift valve opening (about 4-6 mm) for low speed, low load engine operation, and a high-lift valve opening (about 8-10 mm) for high speed, high load engine operation. The engine is further equipped with devices for controlling phasing (i.e., relative timing) of opening and closing of the intake and exhaust valves <b>20</b> and <b>18</b>, referred to as variable cam phasing (hereafter ‘VCP’), to control phasing beyond that which is effected by the two-step VLC lift. There is a VCP/VLC system <b>22</b> for the intake valves <b>20</b> and a VCP/VLC system <b>24</b> for the engine exhaust valves <b>18</b>. The VCP/VLC systems <b>22</b> and <b>24</b> are controlled by the control module <b>5</b>, and provide signal feedback to the control module <b>5</b> for example through camshaft rotation position sensors for the intake camshaft (not shown) and the exhaust camshaft (not shown).

The intake and exhaust VCP/VLC systems <b>22</b> and <b>24</b> have limited ranges of authority over which opening and closing of the intake and exhaust valves <b>18</b> and <b>20</b> can be controlled. VCP systems can have a range of phasing authority of about 60°-90° of cam shaft rotation, thus permitting the control module <b>5</b> to advance or retard opening and closing of one of the intake and exhaust valves <b>20</b> and <b>18</b>. The range of phasing authority is defined and limited by the hardware of the VCP and the control system which actuates the VCP. The intake and exhaust VCP/VLC systems <b>22</b> and <b>24</b> may be actuated using one of electro-hydraulic, hydraulic, and electric control force, controlled by the control module <b>5</b>.

The engine <b>10</b> includes a fuel injection system, comprising a plurality of high-pressure fuel injectors <b>28</b> each adapted to directly inject a mass of fuel into one of the combustion chambers <b>16</b>, in response to a signal from the control module <b>5</b>. The fuel injectors <b>28</b> are supplied pressurized fuel from a fuel distribution system (not shown).

The engine <b>10</b> includes a spark-ignition system by which spark energy is provided to a spark plug <b>26</b> for igniting or assisting in igniting cylinder charges in each of the combustion chambers <b>16</b> in response to a signal (‘IGN’) from the control module <b>5</b>.

The engine <b>10</b> is equipped with various sensing devices for monitoring engine operation, including a crank sensor <b>42</b> having output (‘RPM’) and operative to monitor crankshaft rotational position, i.e., crank angle and speed, a combustion sensor <b>30</b> adapted to monitor combustion and an exhaust gas sensor <b>40</b> adapted to monitor exhaust gases, typically an air/fuel ratio sensor. The combustion sensor <b>30</b> comprises a sensor device operative to monitor a state of a combustion parameter and is depicted as a cylinder pressure sensor operative to monitor in-cylinder combustion pressure. The output of the combustion sensor <b>30</b> and the crank sensor <b>42</b> are monitored by the control module <b>5</b> which determines combustion phasing, i.e., timing of combustion pressure relative to the crank angle of the crankshaft <b>12</b> for each cylinder <b>15</b> for each combustion cycle. The combustion sensor <b>30</b> can also be monitored by the control module <b>5</b> to determine a mean-effective-pressure (hereafter ‘IMEP’) for each cylinder <b>15</b> for each combustion cycle. Preferably, the engine <b>10</b> and control module <b>5</b> are mechanized to monitor and determine states of IMEP for each of the engine cylinders <b>15</b> during each cylinder firing event. Alternatively, other sensing systems can be used to monitor states of other combustion parameters within the scope of the disclosure, e.g., ion-sense ignition systems, and non-intrusive cylinder pressure sensors.

The control module <b>5</b> executes algorithmic code stored therein to control the aforementioned actuators to control engine operation, including throttle position, spark timing, fuel injection mass and timing, intake and/or exhaust valve timing and phasing, and EGR valve position to control flow of recirculated exhaust gases. Valve timing and phasing includes NVO and lift of exhaust valve reopening (in an exhaust re-breathing strategy). The control module <b>5</b> is adapted to receive input signals from an operator (e.g., a throttle pedal position and a brake pedal position) to determine an operator torque request and from the sensors indicating the engine speed and intake air temperature, and coolant temperature and other ambient conditions.

The control module <b>5</b> is preferably a general-purpose digital computer comprising a microprocessor or central processing unit, storage mediums comprising non-volatile memory including read only memory and electrically programmable read only memory, random access memory, a high speed clock, analog to digital and digital to analog circuitry, and input/output circuitry and devices and appropriate signal conditioning and buffer circuitry. The control module has a set of control algorithms, comprising resident program instructions and calibrations stored in the non-volatile memory and executed to provide the respective functions of each computer. The algorithms are preferably executed during preset loop cycles. Algorithms are executed by the central processing unit and are operable to monitor inputs from the aforementioned sensing devices and execute control and diagnostic routines to control operation of the actuators, using preset calibrations. Loop cycles may be executed at regular intervals, for example each 3.125, 6.25, 12.5, 25 and 100 milliseconds during ongoing engine and vehicle operation. Alternatively, algorithms may be executed in response to occurrence of an event.

In operation, the control module <b>5</b> monitors inputs from the aforementioned sensors to determine states of engine parameters. The control module <b>5</b> executes algorithmic code stored therein to control the aforementioned actuators to form the cylinder charge, including controlling throttle position, spark-ignition timing, fuel injection mass and timing, EGR valve position to control flow of recirculated exhaust gases, and intake and/or exhaust valve timing and phasing on engines so equipped. The control module <b>5</b> can operate to turn the engine on and off during ongoing vehicle operation, and can operate to selectively deactivate a portion of the combustion chambers or a portion of the valves through control of fuel and spark and valve deactivation.

The exhaust aftertreatment system <b>70</b> in this embodiment comprises a three-way catalytic converter <b>80</b> (hereafter ‘TWC’), a catalyzed NOx adsorber <b>60</b> (hereafter ‘LNT device’), a flow diverter valve <b>72</b>, and a bypass tube <b>74</b>. The aftertreatment system <b>70</b> is fluidly connected to the internal combustion engine <b>10</b> and signally and operatively connected to the control module <b>5</b>. The diverter valve <b>72</b> is fluidly connected to the engine <b>10</b>, the three-way catalytic converter <b>80</b>, and the bypass tube <b>74</b>, and operatively connected to the control module <b>5</b>. The flow diverter valve <b>72</b> can control flow of the exhaust gas feedstream directly though the three-way catalytic converter <b>80</b> to the LNT device <b>60</b> and can divert exhaust gas feedstream around the three-way catalytic converter <b>80</b> and directly into the catalyzed LNT device <b>60</b> from the engine <b>10</b>.

The three-way catalytic converter <b>80</b> is fluidly connected downstream from the diverter valve <b>72</b> and upstream from the LNT device <b>60</b>. The LNT device <b>60</b> is fluidly connected downstream from the three-way catalytic converter <b>80</b> and the bypass tube <b>74</b>. The bypass tube <b>74</b> is fluidly connected on a first end <b>82</b> to the diverter valve <b>72</b> and fluidly connected on a second end <b>84</b> to the input of the LNT device <b>60</b>. Exhaust gas sensors <b>50</b> and <b>52</b> are disposed to monitor the feedstream and signally connected to the control module. The exhaust gas sensors <b>50</b> and <b>52</b> can be any of air/fuel sensors, NOx sensors, and oxygen sensors.

The LNT device <b>60</b> is operative to adsorb nitrides of oxygen in the exhaust gas feedstream, with the amount of adsorption based upon temperature, flowrate, and air/fuel ratio of the exhaust gas feedstream and amount of nitrides of oxygen already adsorbed thereon.

The LNT device <b>60</b> preferably comprises a substrate having a washcoat containing catalytically active material. The substrate preferably comprises a monolithic element formed from cordierite with a cell density that is preferably 64-96 cells per square centimeter (400 to 600 cells per square inch), and a wall thickness of three to seven mils. The cells of the substrate comprise flow passages through which exhaust gas flows to contact the catalytically active materials of the washcoat to effect adsorption and desorption of nitrides of oxygen, oxygen storage, and oxidization and reduction of constituents of the exhaust gas feedstream. The washcoat preferably contains alkali elements, e.g., Li, Na, and K, and alkali earth metal compounds, e.g., Ba, Mg, Ca, and Sr, and/or lanthanum group metals e.g., La, and Ce operative to store NOx as nitrides of oxygen that are generated during engine operation that is lean of stoichiometry. The washcoat can also contain catalytically active materials, i.e., platinum-group metals (hereafter ‘PGM’) comprising Pt, Pd, and Rh, and additives (e.g., Ce, Zr, La). When the exhaust gas feedstream is rich of stoichiometry there are excess reductants and adsorbed nitrides of oxygen are not stable and decompose to release stored NOx. The reductants in the exhaust gas feedstream preferably comprise HC molecules, hydrogen molecules, and CO which are generated when the engine is operated at a rich air/fuel ratio. The washcoat adsorbs nitrides of oxygen during lean engine operation, and desorbs and reduces nitrides of oxygen during engine operation that generates a rich exhaust gas feedstream. The desorbed nitrides of oxygen are reduced by the excess reductants at PGM catalyst sites. The LNT device <b>60</b> can saturate with adsorbed nitrides of oxygen, thus reducing its effectiveness. The LNT device <b>60</b> can be regenerated by desorbing the adsorbed nitrides of oxygen in the presence of the aforementioned reductants by reacting with the reductants to reduce to nitrogen and other inert elements.

As shown in <figref idref="DRAWINGS">FIG. 1</figref>, the aftertreatment system <b>70</b> preferably includes an air/fuel sensor <b>40</b> upstream of the diverter valve <b>72</b>, and sensor <b>52</b> preferably comprising an air/fuel ratio sensor upstream of the LNT device <b>60</b> and downstream of the second end <b>84</b> of the bypass tube <b>74</b>. The aftertreatment system <b>70</b> may also include sensor <b>50</b> comprising a NOx sensor downstream of the LNT device <b>60</b> that is preferably capable to measure air/fuel ratio of the exhaust gas feedstream. Alternatively, sensor <b>50</b> can comprise an air/fuel ratio sensor downstream of the LNT device <b>60</b>. In one embodiment, one NOx sensor and one air/fuel ratio sensor may both be disposed downstream of the LNT device. In an alternate embodiment, a single sensor may be adapted to provide functionality as both a NOx sensor and an air/fuel ratio sensor. The sensors <b>50</b> and <b>52</b> each generate output signals monitored by the control module <b>5</b> which can be used for control and diagnostics of the engine <b>10</b> and the aftertreatment system <b>70</b>.

<figref idref="DRAWINGS">FIG. 2</figref> depicts a method for managing an exhaust gas feedstream from the engine <b>5</b> during ongoing powertrain operations, preferably executed as one or more algorithms in the control module <b>5</b>. When the engine <b>10</b> is running lean of stoichiometry, exhaust is directed through the TWC <b>80</b> to the LNT device <b>60</b> (<b>200</b>). The control module <b>5</b> monitors the sensors, including the NOx sensor <b>50</b>. The NOx sensor <b>50</b> monitors the amount of NOx emissions emitted from the LNT device <b>60</b> and the air/fuel ratio, and signals the control module <b>5</b>. When the NOx sensor <b>50</b> indicates that a predetermined threshold of NOx emissions are emitted from the LNT device <b>60</b>, then the control module <b>5</b> may command LNT regeneration, preferably prior to saturation of the LNT and NOx breakthrough (<b>202</b>). One skilled in the art appreciates that NOx breakthrough can be monitored with a mathematical model in place of or in conjunction with the NOx sensor <b>50</b>. When the model indicates NOx breakthrough, the control module <b>5</b> can command LNT regeneration (<b>202</b>).

During LNT regeneration, the control module <b>5</b> can command the diverter valve <b>72</b> to open (<b>204</b>). Once the diverter valve <b>72</b> is open the exhaust gas feedstream is diverted around the TWC <b>80</b> to the LNT device <b>60</b> (<b>206</b>). By diverting the exhaust gas feedstream around the TWC <b>80</b> the reductants produced by the engine <b>10</b> are free to desorb NOx emissions stored in the LNT device <b>60</b>, thereby decreasing the amount of reductants needed to achieve LNT regeneration and improving fuel efficiency.

At the start of LNT regeneration, the engine <b>10</b> can be commanded to produce an exhaust gas feedstream that is rich of stoichiometry to generate the reductants (<b>208</b>). The LNT regeneration can include injecting a first fuel pulse into the combustion chamber <b>16</b> during the compression stroke. The mass of fuel injected during the first fuel pulse is determined based upon an amount sufficient to operate the engine <b>10</b> to meet the operator torque request. Subsequent fuel pulses can be injected to the combustion chamber <b>16</b> during the combustion cycle. The subsequent fuel pulses can be injected during other strokes of the combustion cycle, e.g., the power stroke or the exhaust stroke, to generate an exhaust gas feedstream having an air/fuel ratio which is rich of stoichiometry to act as reductants to regenerate the LNT device <b>60</b>.

The control module <b>5</b> determines that the LNT device <b>60</b> has been regenerated (<b>210</b>). Concurrent with regeneration and the engine <b>10</b> transitioning operation from lean of stoichiometry to rich of stoichiometry, the control module <b>5</b> can monitor NOx and/or the air/fuel ratio downstream of the LNT device <b>60</b> using sensor <b>50</b> to monitor air/fuel ratio and/or NOx emissions. When the sensor <b>50</b> indicates a predetermined air/fuel ratio or a predetermined NOx threshold, the control module <b>5</b> may determine that regeneration is complete. Alternatively, the control module <b>5</b> can initiate regeneration and operate the engine <b>10</b> rich of stoichiometry for a predetermined time interval. Once the time period elapses, the control module <b>5</b> determines that regeneration is complete, and discontinues operating the engine <b>10</b> rich of stoichiometry.

Once the control module <b>5</b> determines that regeneration is complete the control module <b>5</b> commands the diverter valve <b>72</b> to close (<b>212</b>). Once the diverter valve <b>72</b> is closed, the exhaust gas feedstream is directed through the TWC <b>80</b> device to the LNT device <b>60</b> (<b>214</b>). The control module <b>5</b> may also return engine operation to lean of stoichiometry (<b>200</b>). The control module <b>5</b> continues to monitor the NOx sensor <b>50</b> downstream of the LNT device <b>60</b> to determine when to initiate subsequent LNT regeneration events.

<figref idref="DRAWINGS">FIG. 3</figref> shows a datagraph depicting CO breakthrough out of the TWC <b>80</b>, and LNT NOx efficiency plotted as a function of the OSC for the aftertreatment system <b>70</b>. As the OSC consumes less of the reductants produced by the engine <b>10</b>, CO breakthrough out of the TWC <b>80</b> increases slightly. The CO breakthrough from the TWC <b>80</b> reacts with the adsorbed NOx in the LNT device <b>60</b> increasing NOx efficiency within the LNT device <b>60</b> and decreasing NOx emissions.

The diverter valve <b>72</b> and the bypass tube <b>74</b> can also be used during an engine start and initial engine operation. During an engine start and initial operation, the engine <b>10</b> may operate at stoichiometry or rich of stoichiometry. The exhaust gas feedstream is directed through the TWC <b>80</b> to the LNT device <b>60</b>. As the exhaust gas feedstream passes through the aftertreatment system <b>70</b>, it heats the TWC <b>80</b> to a temperature whereat HC in the exhaust gas feedstream can be oxidized. For the exemplary LNT device <b>60</b>, NOx absorption and reduction occurs most effectively between a temperature range of about 250° C. to 450° C. Once the LNT device <b>60</b> reaches a temperature in the range of 200° C., the control module <b>5</b> can command the engine <b>10</b> to begin operating at lean of stoichiometry. The exhaust gas feedstream may then be diverted around the TWC <b>80</b> for LNT regeneration based upon the NOx indicated downstream of the LNT device <b>60</b>.

Alternatively, during engine starts at stoichiometry or rich of stoichiometry, the exhaust gas feedstream may be diverted around the TWC <b>80</b> until the exhaust gas feedstream heats the LNT device <b>60</b> to a predetermined operating temperature. Diverting the exhaust gas feedstream around the TWC <b>80</b> may be initiated based upon temperature of the TWC <b>80</b>, elapsed time from the engine start, and/or engine cycles. Once the control module <b>5</b> determines that a predetermined temperature, elapsed time, or a predetermined number engine cycles have been achieved, the exhaust gas feedstream may be diverted to the LNT device <b>60</b>. Once the LNT device <b>60</b> reaches the operating temperature the control module <b>5</b> can command the engine <b>10</b> to begin operating at lean of stoichiometry, and the diverter valve <b>72</b> to direct the exhaust gas feedstream through the TWC <b>80</b> to the LNT device <b>60</b> until the control module <b>5</b> initiates LNT regeneration. This method heats the LNT device <b>60</b> to an operating temperature, thereby allowing the engine <b>10</b> to begin operating lean of stoichiometry.

The same methods as described hereinabove are applicable to multiple more powertrain systems including powertrain systems with compression ignition, spark-ignition, homogeneous-charge compression-ignition, and spark-ignition direct-injection engines. The methods describe hereinabove are also applicable to powertrain systems which include electrical machines and internal combustion engines that transition among engine off and engine on states during ongoing powertrain operations. Another exemplary powertrain system comprises a belt alternator starter system (BAS). In the BAS system, an engine can be shut OFF during stops and then started for a battery recharge and multiple acceleration situations. The electrical motor may be used to recharge the battery or to provide torque mechanically through the engine during engine acceleration.

The disclosure has described certain preferred embodiments and modifications thereto. Further modifications and alterations may occur to others upon reading and understanding the specification. Therefore, it is intended that the disclosure not be limited to the particular embodiment(s) disclosed as the best mode contemplated for carrying out this disclosure, but that the disclosure will include all embodiments falling within the scope of the appended claims.